An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head. Petrol and diesel opposed-piston engines have been used mostly in large applications such as ships, military tanks, and factories. Current manufacturers of opposed-piston engines include Cummins, Achates Power, and Fairbanks-Morse Defense (FMDefense).

Compared to contemporary two-stroke engines, which use a conventional design of one piston per cylinder, the advantages of the opposed-piston engine are:

The main drawback was that the two opposing pistons had to be geared together. This added weight and complexity when compared to conventional piston engines, which use a single crankshaft as the power output.

The most common layout was two crankshafts, with the crankshafts geared together (in either the same direction or opposing directions). The Koreyvo, Jumo, and Napier Deltic engines used one piston per cylinder to expose an intake port, and the other to expose an exhaust port. Each piston is referred to as either an intake piston or an exhaust piston, depending on its function in this regard. This layout gives superior scavenging, as gas flow through the cylinder is axial rather than radial, and simplifies design of the piston crowns. In the Jumo 205 and its variants, the upper crankshaft serves the exhaust pistons, and the lower crankshaft the intake pistons. In designs using multiple cylinder banks, each big end bearing serves one inlet and one exhaust piston, using a forked connecting rod for the exhaust piston.

One of the first opposed-piston engines was the 1882 Atkinson differential engine, which has a power stroke on every rotation of the crankshaft (compared with every second rotation for the contemporary Otto cycle engine), but it was not a commercial success.

In 1898, an Oechelhäuser two-stroke opposed-piston engine producing 600 hp (447 kW) was installed at the Hoerde ironworks. This design of engine was also produced under licence by manufacturers including Deutsche Kraftgas Gesellschaft in Germany and William Beardmore & Sons in the United Kingdom.

In 1901, the Kansas City Lightning Balanced Gas and Gasoline Engines were gasoline engines producing 4–25 hp (3–19 kW).

An early opposed-piston car engine was produced by the French company Gobron-Brillié around 1900. On 31 March 1904, a Gobron-Brillié car powered by the opposed-piston engine was the first car ever to exceed 150 km/h with a "World's Record Speed" of 152.54 km/h (95 mph). On 17 July 1904, the Gobron-Brillié car became the first to exceed 100 mph (161 km/h) for the flying kilometre. The engine used a single crankshaft at one end of the cylinders and a crosshead for the opposing piston.